This process relates to a coating process and compositions prepared by this process. The invention specifically relates to iron-based, metallurgical compositions, and more particularly, to metallurgical compositions that contain a binding agent that also provides lubrication during the compaction process used to form a part.
Coating of particles is an important process for modifying particles and the surface properties of the particles. Methods for particle coating include the Wurster process as described in U.S. Pat. Nos. 2,648,609; 3,117,027; and 3,253,944 and more recently in U.S. Pat. Nos. 4,731,195 and 5,085,930 in which particles are fluidized in some manner and the fluidized particles are then spray coated with coating materials dissolved in various solvents or the coating materials are sprayed onto the core particles as a low viscosity melt; spray coating is also done in which the particles and the coating material is passed through a suitable atomizer. One example of this method is shown in U.S. Pat. No. 4,675,140 in which the coating material is a melted polymer. An interesting method is presented in U.S. Pat. No. 5,262,240 in which the coating is effected by mixing the particles with a latex and drying the resulting mixture. In this process a coated aggregate is produced. Well known methods for the coating of particles with thin organic layers can use a surface active agent such as organosilanes or fluorocarbons to modify the surface properties. In this method the particles are soaked in a solution and the surface active agent reacts with the particle. U.S. Pat. No. 4,994,326 is one example of such a process and the materials resulting form the treatment. Finally, a preferred method for particle coating is tumble blending or high shear hot blending. A number of patents describe this process. U.S. Pat. No. 4,233,387 describes a process wherein electrophotographic carriers are treated with thermoplastic resins about 325xc2x0 F. The resulting mixture is then cooled, ground, to an appropriate size and used to charge toners in a photocopier. U.S. Pat. No. 4,774,139 describes a process for coating paraffin onto thermoplastic hot melt resin. U.S. Pat. No. 4,885,175 describes a method for coating a sweetener with a molten wax, cooling the mixture, and grinding the cooled mass to the desired size. Finally, as an example in this hot melt blending process U.S. Pat. No. 4,1356,566 describes the coating of iron powders with a polymer and additives by mixing the ingredients in a high shear mixer at temperatures above the melting point of the polymer coating material.
In all the above cases the processes are lacking in a number of important aspects. In the Wurster-like processes, the methods all involve having a particle that is fluidizable. Typically, this is a particle with at least an average size of 50 microns. Further, in the Wurster-like process if solutions are used as the coating vehicle, the solvent, water or organic solvent, must be removed by drying. This is tedious for water solutions and dangerous for flammable liquids. In direct atomization methods there is the difficulty of separation of the coated and uncoated particles. While U.S. Pat. No. 4,675,140 describes a method for particle separation, this technique is not universally applicable to all materials. There are two problems that hinder the application of hot blending methods, in which the processing temperature is above the melting point. First, the agglomeration is an undesired side effect for this process. For many applications subsequent grinding and classification processing can be difficult or too expensive. Secondly, operation of the process above the melting point does result in higher energy costs, which is always undesirable. U.S. Pat. No. 5,147,722 teaches a method in which coating of the particles can be done below the melting point of the polymer binder. Under the conditions of this process with high shear mixing and high applied pressure, the particles are coated, but a web-like matrix is formed. However, for various compositions agglomeration is not desired and the use of high shear mixers and high pressure adds extra capital and operating cost. U.S. Pat. No. 5,236,649 also teaches that particle coating can be done at a temperature lower than the melting point of the coating material. However, as in U.S. Pat. No. 5,147,722, the process requires high shear mixing to obtain a good coating.
The use of coated particles has use in decorative masonry, proponents for oil wells, taste masking in the food and pharmaceutical industries and in the powder metallurgy industry. The powder metallurgy industry has developed metal-based powder compositions, generally iron-based powders, that can be processed into integral metal parts having various shapes and sizes for uses in various industries, including the automotive and electronics industries. One processing technique for producing the parts from the base powders is to charge the powder into a die cavity and compact the powder under high pressures. The resultant green compact is then removed from the die cavity and sintered to form the final part.
Industrial usage of metal parts manufactured by the compaction and sintering of metal powder compositions is expanding rapidly into a multitude of areas. Manufacture of these parts with metal powder compositions provides substantial benefits in comparison to having to use a molten alloy in the manufacturing process. For instance, the metal powder compositions allow for the manufacturing process to proceed with just a high pressure compaction die machine and a sintering oven. The different parts are made by simply replacing the compaction die. Further, there is no need to handle molten alloys.
In the manufacture of such parts, iron or steel particulate powders are often admixed with at least one other alloying element that is also in particulate form. These alloying elements permit the attainment of higher strength and other mechanical properties in the final sintered part. The alloying elements typically differ from the base iron or steel powders in particle size, shape and density. For example, the average particle size of the iron-based powders is typically about 70-100 microns, or more, while the average particle size of most alloying ingredients is less than about 20 microns, more often less than about 15 microns, and in some cases less than about 5 microns. The alloying powders are purposely used in such a finely-divided state to promote rapid homogenization of the alloy ingredients by solid-state diffusion during the sintering operation.
The presence of the different particle size materials leads to problems such as segregation and dusting upon transportation, storage and use. The iron and alloy element powders are initially blended into a homogeneous powder. The dynamics of handling the powder mixture during storage and transfer cause the smaller alloying powder particles to migrate through the interstices of the iron-based powder matrix, resulting in a loss of homogeneity of the mixture, or segregation. On the other hand, air currents that can develop within the powder matrix as a result of handling can cause the smaller alloying powders, particularly if they are less dense than the iron powders, to migrate upwardly. If these buoyant forces are high enough, some of the alloying particles can, in the phenomenon known as dusting, escape the mixture entirely, resulting in a decrease in the concentration of the alloy element.
Various organic binding agents have been used to bind or xe2x80x9cgluexe2x80x9d the finer alloying powder to the coarser iron-based particles to prevent segregation and dusting for powders to be compacted at ambient temperatures. For example, U.S. Pat. No. 4,483,905 to Engstrxc3x6m teaches the use of a binding agent that is broadly described as being of xe2x80x9ca sticky or fat characterxe2x80x9d in an amount up to about 1% by weight of the powder composition. U.S. Pat. No. 4,676,831 to Engstrxc3x6m discloses the use of certain tall oils as binding agents. Also, U.S. Pat. No. 4,834,800 to Semel discloses the use of certain film-forming polymeric resins that are insoluble or substantially insoluble in water as binding agents.
Various other types of binding agents are set forth in the patent literature. Polyalkylene oxides having molecular weights of at least about 7000 are disclosed as binding agents in U.S. Pat. No. 5,298,055. Combinations of dibasic organic acid and one or more additional components such as solid polyethers, liquid polyethers, and acrylic resins as binding agents are disclosed in U.S. Pat. No. 5,290,336. Binding agents that can be used with high temperature compaction lubricants are disclosed in U.S. Pat. No. 5,368,630.
U.S. Pat. No. 5,480,469 (xe2x80x9c469 patentxe2x80x9d) provides a brief review of the use of binding agents in the powder metallurgy industry. The 469 patent notes that it is important to have not only a powder composition that has the alloying powder adhered to the iron-based powder by way of the binding agent, but to also have a lubricant present to achieve adequate compressibility of the powder composition within the die and to decrease the forces required to remove the part from the die. The 469 patent discusses various references that disclose the use of a binding agent in conjunction with a lubricant powder, such as a metal soap, to be blended with the iron-based and alloying powders. This blend is then heated and mixed to melt the binding agent and the lubricant and to bind the alloy powder to the iron-based powder. This mixture is then cooled to form the final composition. The 469 patent discloses an improvement to this type of technology by using a diamide wax as the binding agent whereby a metal soap lubricant is not required.
The presence of a binding agent should not adversely affect the compressibility of the powder metallurgical composition. The xe2x80x9ccompressibilityxe2x80x9d of a powder blend is a measure of its performance under various conditions of compaction. In the art of powder metallurgy, a powder composition is generally compacted under great pressure in a die, and the compacted xe2x80x9cgreenxe2x80x9d part is then removed from the die and sintered. It is recognized in this art that the density, and usually the strength, of this green part vary directly with the compaction pressure. In terms of xe2x80x9ccompressibilityxe2x80x9d, one powder composition is said to be more compressible than another if, at a given compaction pressure, it can be pressed to a higher green density, or alternatively, if it requires less compaction pressure to attain a specified green density. If the binding agent has good xe2x80x9cinternalxe2x80x9d lubrication characteristics, it will enhance the compressibility of the powder composition and result in a higher green density at a given compaction pressure.
Therefore, a need exists for a coating process that can provide a simple and inexpensive method for coating a variety of particles. In the powder metallurgical industry a specific need exists for a metallurgical composition that contains the alloying powder(s) bonded to the metal-based powder where that composition can be prepared in a solvent-less process. The binding agent used in the metallurgical composition should function to decrease the amount of dusting and/or segregation of the alloying powder(s) and also not adversely effect the compressibility of the composition.
The present invention provides an improved method for particle coating that uses a low shear, low temperature method to produce non-agglomerated, coated particles. In one embodiment, the present invention provides improved powder metallurgical compositions containing a major amount of a metal-based powder bound to a minor amount of at least one alloying powder.
The particulate powders that can be coated in accordance with the present invention include metal powders such as iron, copper, nickel, cobalt, chromium, aluminum, zinc, silicon, manganese, silver, gold, platinum, palladium, titanium, their alloys and blends thereof; inorganic oxides such as alumina, silica, and titania; inorganic compounds such as common table salt, peroxide bleaches, bath salts, calcium chloride, and inorganic fertilizers such as potash; and solid organic compounds such polymers, acids and bases. The core particles can be in any-form such as beads, flakes, fibers, and acicular particles wherein at least one dimension, on a number average, is in the range of 10 microns to 1 cm, preferably in the range of 20 microns to 0.75 cm and most preferably in the range of 25-10,000 microns.
The coating material, referred to herein also as a binding agent or material, particularly with respect to powder metallurgy compositions, can be a low melting, solid polymer or wax, e.g., a polymer or wax having a softening temperature of below 200xc2x0 C. (390xc2x0 F.), preferably below 150xc2x0 C. (300xc2x0 F.), and more preferably between about 65-95xc2x0 C. (150-250xc2x0 F.). Examples of solid polymeric binding agents include polyesters, polyethylenes, epoxies, and urethanes. Examples of waxes include paraffins, ethylene bisstearamides, and cotton seed waxes. The solid binding agent can also be polyolefins with weight average molecular weights below 3,000, and hydrogenated vegetable oils that are C14-24 alkyl moiety triglycerides and derivatives thereof, including hydrogenated derivatives, e.g. cottonseed oil, soybean oil, jojoba oil, and blends thereof. The solid coating material is preferably reduced to an average particle size in at least one dimension of less than 200 or 100 microns, and preferably in the range of between 0.01 and 50 microns, more preferably between 0.01 and 20 microns.
The coating process of the present invention can be a xe2x80x9cdryxe2x80x9d bonding process that does not require a solvent for the binding agent. The process used involves the mixing of a suitable binding agent in the preferred particle size range, with the core particles, and any alloying particles or any additives at ambient or elevated temperatures. The blend is then gently mixed using a conventional mixer under low shear conditions. The mixture is preferably heated to at least 120xc2x0 F. (49xc2x0 C.) and preferably to a temperature below the melting point of the binding agent, blended, and then cooled to provide the final product.
The powder metallurgy compositions of the present invention are prepared by mixing the metal-based powder with the alloying powder(s) at ambient or elevated temperatures and blending the binding agent with those powders either at ambient or elevated temperatures. During the blending process the binding agent is contacted with the metal-based and alloying powders at temperatures of at least about 120xc2x0 F. (49xc2x0 C.). This blended composition can then be cooled to ambient conditions. The temperature of the blending process is preferably conducted at a bulk powder temperature that is below the melting point of the binding agent.
The preferred binding agent for powder metallurgy applications is polyethylene wax. The polyethylene wax is preferably introduced into the mixture of the metal-based and alloying powders in its solid state. If introduced in its solid state, it can be used in various forms such as spheres, fibers, or flakes. Particularly advantageous results are obtained by using a polyethylene wax in the form of spheres having an average particle size of below about 50, and preferably below about 30, microns.
The present invention relates to an improved process for the formation of coated particles, improved metallurgical powder compositions, methods for the preparation of those compositions, and methods for using those compositions to make compacted parts. The powder metallurgy compositions comprise a metal-based powder, preferably an iron-based metal powder, in admixture with at least one alloying powder, and a binding agent for adhering the alloying powder to the metal-based powder. The preferred binding agent for powder metallurgy applications is a polyethylene wax having a weight average molecular weight of below about 4000, more preferably below about 2000. It has been found that the use of the polyethylene wax as a binding agent for the metallurgical powder composition provides superior dusting/segregation resistance and also provides improved strength and ejection performance of the green compact.
The particles that can be coated in accordance with the present invention include metal powders such as iron, copper, nickel, cobalt, chromium, aluminum, zinc, silicon, manganese, silver, gold, platinum, palladium, titanium, their alloys and blends thereof; inorganic oxides such as alumina, silica, and titania; inorganic compounds such as common table salt, peroxide bleaches, bath salts, calcium chloride, and inorganic fertilizers such as potash; and solid organic compounds such polymers, acids and bases. The core particles can be in any form such as beads, flakes, fibers, and acicular parties wherein at least one dimension, on a number average, is in the range of 10 microns to 1 cm, preferably in the range of 20 microns to 0.75 cm and most preferably in the range of 25-10,000 microns.
The powder metallurgy compositions of the present invention comprise metal powders of the kind generally used in the powder metallurgy industry, such as iron-based powders and nickel-based powders. The metal powders constitute a major portion of the metallurgical powder composition, and generally constitute at least about 80 weight percent, preferably at least about 90 weight percent, and more preferably at least about 95 weight percent of the composition.
Examples of xe2x80x9ciron-basedxe2x80x9d powders, as that term is used herein, are powders of substantially pure iron, powders of iron pre-alloyed with other elements (for example, steel-producing elements) that enhance the strength, hardenability, electromagnetic properties, or other desirable properties of the final product, and powders of iron to which such other elements have been diffusion bonded.
Substantially pure iron powders that can be used in the invention are powders of iron containing not more than about 1.0% by weight, preferably no more than about 0.5% by weight, of normal impurities. Examples of such highly compressible, metallurgical-grade iron powders are the ANCORSTEEL 1000 series of pure iron powders, e.g. 1000, 1000B, and 1000C, available from Hoeganaes Corporation, Riverton, N.J. For example, ANCORSTEEL 1000 iron powder, has a typical screen profile of about 22% by weight of the particles below a No. 325 sieve (U.S. series) and about 10% by weight of the particles larger than a No. 100 sieve with the remainder between these two sizes (trace amounts larger than No. 60 sieve). The ANCORSTEEL 1000 powder has an apparent density of from about 2.85-3.00 g/cm3, typically 2.94 g/cm3. Other iron powders that can be used in the invention are typical sponge iron powders, such as Hoeganaes"" ANCOR MH-100 powder.
The iron-based powder can incorporate one or more alloying elements that enhance the mechanical or other properties of the final metal part. Such iron-based powders can be powders of iron, preferably substantially pure iron, that has been pre-alloyed with one or more such elements. The pre-alloyed powders can be prepared by making a melt of iron and the desired alloying elements, and then atomizing the melt, whereby the atomized droplets form the powder upon solidification.
Examples of alloying elements that can be pre-alloyed with the iron powder include, but are not limited to, molybdenum, manganese, magnesium, chromium, silicon, copper, nickel, gold, vanadium, columbium (niobium), graphite, phosphorus, aluminum, and combinations thereof. The amount of the alloying element or elements incorporated depends upon the properties desired in the final metal part. Pre-alloyed iron powders that incorporate such alloying elements are available from Hoeganaes Corp. as part of its ANCORSTEEL line of powders.
A further example of iron-based powders are diffusion-bonded iron-based powders which are particles of substantially pure iron that have a layer or coating of one or more other metals, such as steel-producing elements, diffused into their outer surfaces. Such commercially available powders include DISTALOY 4600A diffusion bonded powder from Hoeganaes Corporation, which contains about 1.8% nickel, about 0.55% molybdenum, and about 1.6% copper, and DISTALOY 4800A diffusion bonded powder from Hoeganaes Corporation, which contains about 4.05% nickel, about 0.55% molybdenum, and about 1.6% copper.
A preferred iron-based powder is of iron pre-alloyed with molybdenum (Mo). The powder is produced by atomizing a melt of substantially pure iron containing from about 0.5 to about 2.5 weight percent Mo. An example of such a powder is Hoeganaes"" ANCORSTEEL 85HP steel powder, which contains about 0.85 weight percent Mo, less than about 0.4 weight percent, in total, of such other materials as manganese, chromium, silicon, copper, nickel, molybdenum or aluminum, and less than about 0.02 weight percent carbon. Another example of such a powder is Hoeganaes"" ANCORSTEEL 4600V steel powder, which contains about 0.5-0.6 weight percent molybdenum, about 1.5-2.0 weight percent nickel, and about 0.1-.25 weight percent manganese, and less than about 0.02 weight percent carbon.
Another pre-alloyed iron-based powder that can be used in the invention is disclosed in U.S. Pat. No. 5,108,493, entitled xe2x80x9cSteel Powder Admixture Having Distinct Pre-alloyed Powder of Iron Alloys,xe2x80x9d which is herein incorporated in its entirety. This steel powder composition is an admixture of two different pre-alloyed iron-based powders, one being a pre-alloy of iron with 0.5-2.5 weight percent molybdenum, the other being a pre-alloy of iron with carbon and with at least about 25 weight percent of a transition element component, wherein this component comprises at least one element selected from the group consisting of chromium, manganese, vanadium, and columbium. The admixture is in proportions that provide at least about 0.05 weight percent of the transition element component to the steel powder composition. An example of such a powder is commercially available as Hoeganaes"" ANCORSTEEL 41 AB steel powder, which contains about 0.85 weight percent molybdenum, about 1 weight percent nickel, about 0.9 weight percent manganese, about 0.75 weight percent chromium, and about 0.5 weight percent carbon.
Other iron-based powders that are useful in the practice of the invention are ferromagnetic powders. An example is a powder of iron pre-alloyed with small amounts of phosphorus.
The iron-based powders that are useful in the practice of the invention also include stainless steel powders. These stainless steel powders are commercially available in various grades in the Hoeganaes ANCOR(copyright) series, such as the ANCOR(copyright) 303L, 304L, 316L, 410L, 430L, 434L, and 409Cb powders.
The particles of iron or pre-alloyed iron can have a weight average particle size as small as one micron or below, or up to about 850-1,000 microns, but generally the particles will have a weight average particle size in the range of about 10-500 microns. Preferred are iron or pre-alloyed iron particles having a maximum weight average particle size up to about 350 microns; more preferably the particles will have a weight average particle size in the range of about 25-150 microns, and most preferably 80-150 microns.
The metal powder used in the present invention can also include nickel-based powders. Examples of xe2x80x9cnickel-basedxe2x80x9d powders, as that term is used herein, are powders of substantially pure nickel, and powders of nickel pre-alloyed with other elements that enhance the strength, hardenability, electromagnetic properties, or other desirable properties of the final product. The nickel-based powders can be admixed with any of the alloying powders mentioned previously with respect to the iron-based powders. Examples of nickel-based powders include those commercially available as the Hoeganaes ANCORSPRAY(copyright) powders such as the N-70/30 Cu, N-80/20, and N-20 powders.
The metal-based powder can also include any combination of the described metal-based powders.
The metallurgical powder compositions of the present invention also include a minor amount of at least one alloying powder. As used herein, xe2x80x9calloying powdersxe2x80x9d refers to materials that are capable of alloying with the metal-based powder upon sintering. The alloying powders that can be admixed with metal-based powders of the kind described above are those known in the metallurgical arts to enhance the strength, hardenability, electromagnetic properties, or other desirable properties of the final sintered product. Steel-producing elements are among the best known of these materials. Specific examples of alloying materials include, but are not limited to, elemental molybdenum, manganese, chromium, silicon, copper, nickel, tin, vanadium, columbium (niobium), metallurgical carbon (graphite), phosphorus, aluminum, sulfur, and combinations thereof. Other suitable alloying materials are binary alloys of copper with tin or phosphorus; ferro-alloys of manganese, chromium, boron, phosphorus, or silicon; low-melting ternary and quaternary eutectics of carbon and two or three of iron, vanadium, manganese, chromium, and molybdenum; carbides of tungsten or silicon; silicon nitride; and sulfides of manganese or molybdenum.
The alloying powders are in the form of particles that are generally of finer size than the particles of metal powder with which they are admixed. The alloying particles generally have a weight average particle size below about 100 microns, preferably below about 75 microns, more preferably below about 30 microns, and most preferably in the range of about 5-20 microns. The amount of alloying powder present in the composition will depend on the properties desired of the final sintered part. Generally the amount will be minor, up to about 5% by weight of the total powder composition weight, although as much as 10-15% by weight can be present for certain specialized powders. A preferred range suitable for most applications is about 0.25-4.0% by weight of the total powder composition.
The binding agent of the present invention can include solid, low melting polymers or waxes having a softening temperature below about 200xc2x0 C. (390xc2x0 F.), preferably below about 150xc2x0 C. (300xc2x0 F.), more preferably between about 50xc2x0-110xc2x0 C. (125-225xc2x0 F.), and even more preferably between 65xc2x0-95xc2x0 C. (150xc2x0-200xc2x0 F.). Examples of polymeric binding agents include polyesters, polyethylenes, epoxies, and urethanes. Examples of waxes include paraffins, ethylene bisstearamide (ACRAWAX), and cotton seed wax. The binding agent can also include solid polyolefins with weight average molecular weights below 3,000, and solid hydrogenated vegetable oils that can generally be described as triglycerides having C14-24 side chains, and derivatives thereof, including hydrogenated derivatives, such as cotton seed oil, soybean oil, and jojoba oils, and blends thereof.
The binding agent is preferably reduced to an average particle size in at least one dimension of less than 200 microns, and preferably less than 100 microns, with a preferred range of between 0.01 and 50 microns and most preferably in the range of between 0.01 and 20 microns. Therefore, particles in the form of spheres, acicular beads, flakes, or fibers are preferred. Methods of preparing the binder material to get a small particle size includes grinding, crushing, spray drying, melt atomization, extrusion, shaving, and direct reaction. Most preferably melt atomization is used to prepare the binder material in the size ranges listed above. Additional additives can be added to the binding material as needed such as pigments, other metals, inorganic compounds such as salts, graphite, or carbon black, inorganic oxides such as aluminate, silica, and titania.
A preferred binding agent of the present invention particularly for powder metallurgy applications is a solid polyethylene wax having a weight average molecular weight of below about 4000, preferably about 2000 or below, and generally from about 100 to about 4000, and even more preferably from about 500 to about 2000. Suitable polyethylene waxes are commercially available from Petrolite Specialty Polymers Group as the Polywax series, such as Polywax 500 and Polywax 2000. The polyethylene wax preferably has a melt viscosity in the range of 1 to about 500 cps, more preferably between about 3 and 50 cps. The melting point of the polyethylene wax is preferably between 50xc2x0 C. and 200xc2x0 C., more preferably between 75xc2x0 C. and 130xc2x0 C.
A preferred average particle size for the binding agent, such as polyethylene wax, for powder metallurgy applications, is between about 1 and about 50, and even more preferably between about 1 to about 25, microns to aid in the contact between the binding agent with the iron and alloying powders during the blending process. Spherical particles having sizes above these ranges can be used, but it has been found that the temperature of the blending process should be increased in such cases to ensure adequate bonding. If the particulate binding agent is not spherical, at least one dimension of the particles is within the stated ranges for the spherical particles. The particle size of the binding agents can be determined by such methods as laser diffraction techniques. The particle size of the binding agents can be reduced to these ranges by spray atomization techniques commonly known in the industry.
The metallurgical powder composition can be prepared by various blending techniques. Common to all techniques is that the blending of the polymer or wax binding agent with the metal-based and alloying powders is conducted at a powder blend temperature of at least about 27xc2x0 C., preferably at least about 50xc2x0 C., generally in the range between about 50-190xc2x0 C., more preferably between about 65-90xc2x0 C.
In certain situations, and particularly when using the binding agent in the form of solid flakes or lower particle size spheres, it is preferred to blend the binding agent with the metal-based and alloying powders at a temperature below the melting point of the binding agent to improve the properties of the green compact and to limit the ejection forces required to remove the compact from the die cavity. Thus, in certain situations, it is preferred to blend the solid binding agent with the metal-based and alloying powders at a temperature between about 3-35 Celcius degrees, preferably between about 5-30 Celcius degrees, and more preferably between about 8-25 Celcius degrees, below the melting point of the binding agent. For instance, it has been found that beneficial properties are obtained when using polyethylene having a melting point of about 88xc2x0 C., a Mw of about 500, and a weight average particle size of about 20 xcexcm to blend the polyethylene material with the metal-based and alloying powders at a temperature of about 65xc2x0 C.
The metal-based powder can be initially blended with the alloying powder(s) to form a homogeneous mixture at ambient conditions or at the elevated blending temperature. The binding agent, either partially or fully pre-heated, can then be blended with the metal-based and alloying powders in an appropriate vessel in which the temperature of the powder blend can be maintained at the desired level for a time sufficient to contact a substantial, if not entire, portion of the metal-based and alloying powders. The blending of the binding agent is preferably continued until there is obtained a homogeneous mixture. Alternatively, the binding agent can be mixed with the metal-based and alloying powders at the onset, and this mixture can then be heated to the appropriate chosen blending temperature and mixing is conducted at that temperature or temperature range until there is obtained a homogeneous blend. In either process, the blended composition is then cooled to ambient temperature with optional intermittent or continuous mixing.
The concentration of the binding agent in the metallurgical composition (containing the metal-based and alloying powders along with other lubricants, etc.) is in the range of from about 0.05 to about 2, preferably from about 0.25 to about 1.5, and more preferably from about 0.5 to about 1, percent by weight. Concentrations of the binding agent below these levels do not result in effective bonding between the alloying powder and the metal-based powder, and concentrations above these levels generally result in poorer green density and strength properties.
Following the blending of the binding agent into the metallurgical composition, and preferably after that composition has been cooled to an extent, typically to at least below the melting point of the binding agent, and preferably below about 65xc2x0 C., more preferably below about 50xc2x0 C. and more preferably below about 40xc2x0 C., and commonly when the composition is at room temperature, a conventional lubricant can optionally be added and mixed until a homogeneous composition is obtained. The amount of lubricant added can range from about 0.01 to about 2, preferably from about 0.05 to about 1 percent by weight of the final metallurgical composition. Typical lubricants include stearate compounds, such as lithium, zinc, manganese, and calcium stearates commercially available from Witco Corp.; waxes such as ethylene bis-stearamides and polyolefins commercially available from Shamrock Technologies, Inc.; mixtures of zinc and lithium stearates commercially available from Alcan Powders and Pigments as Ferrolube M, and mixtures of ethylene bis-stearamides with metal stearates such as Witco ZB-90. metal stearates and synthetic waxes such as xe2x80x9cACRAWAXxe2x80x9d or xe2x80x9cPM 100xe2x80x9d available from Glyco Chemical Company.
The metallurgical powder compositions as described above can then be compacted in a die to form a metal part in accordance with conventional practices. The resulting green compact can then be sintered in accordance with conventional practices.